Why Use !boolean_variable Over boolean_variable == false

Question

A comment on this question: Checking if a method returns false: assign result to temporary variable, or put method invocation directly in conditional? says that you should use !boolean instead of boolean == false when testing conditions. Why? To me boolean == false is much more natural in English and is more explicit. I apologise if this is just a matter of style, but I was wondering if there was some other reason for this preference of !boolean?

Answer 1

When I see a line like if (!lateForMeeting()), I read that as "If not late for meeting", which is quite straight-forward to understand, as opposed to if (lateForMeeting() == false) which I'd read as "If the fact that I'm late for meeting is false".

They're identical in meaning, but the former is closer to how the equivalent English sentence would be constructed.

Answer 2

Writing == false and == true is redundant. It can be taken to arbitrary extremes, too. If you start writing

if (condition == false) { ... }

Then why not

if ((condition == false) == true) { ... }

Or why not

if ((someExp == anotherExp) == true) { ... }

The moral of this story is that if condition is a boolean expression, then you don't need to add == false; that's what operator ! is for ;)

Answer 3

In C and some similar languages, comparing boolean expressions for equality to false or true is a dangerous habit.

In C any scalar expression (numeric or pointer) can be used in a boolean context, for example as the condition of an if statement. The C rule is that if (cond) is equivalent to if (cond != 0) -- i.e., zero is false, and any non-zero value is true. If cond is of pointer type, 0 is treated as a null pointer constant; if (ptr) means if (ptr != NULL).

This means that

if (cond)

and

if (cond == true)

do not mean the same thing. The first is true if cond is non-zero; the second is true only if it's equal to true, which in C (if you have #include <stdbool.h>) is simply 1.

For example, the isdigit() function declared in <ctype.h> returns an int value: non-zero if the argument is a digit, 0 if it isn't a digit. It can return 42 to indicate that the condition is true. Comparing 42 == true will fail.

It happens that 0 is the only value considered to be false, so comparison for equality to false will work; if (!cond) and if (cond == false) do the same thing. But if you're going to take advantage of that, you have to remember that comparing to false is ok, and comparing to true is not. Worse yet, comparing to true will work most of the time (for example, the equality and relational operators always yield either 0 or 1). This means that any bugs you introduce by using this still could be difficult to track down. (Don't worry, they'll show up as soon as you demo the code to an important client.)

C++ has slightly different rules; for example, its bool type is a bit more tightly integrated into the language, and if (cond) converts cond to type bool. But the effect is (mostly) the same.

Some other languages have what one might call better behaved booleans, such that cond == true and cond == false (or whatever the syntax happens to be) is safe. Even so, every language I've seen has a not or ! operator; it's there, so you might as well use it. Using cond == false rather than !cond or not cond does not, in my opinion, improve readability. (It's true that the ! character can be difficult to see at a glance; I sometimes add a space after the ! to avoid this.)

And often you can avoid the issue and improve clarity by rearranging the code slightly. For example, rather than:

if (!cond) {
    do_this();
}
else {
    do_that();
}

you might write:

if (cond) {
    do_that();
}
else {
    do_this();
}

That's not always better, but it doesn't hurt to look for opportunities where it is.

Summary: In C and C++, equality comparisons to true and false are dangerous, overly verbose, and poor style. In many other languages, such comparisons might not be dangerous, but they're still overly verbose and poor style.

Answer 4

The two are functionally identical, so which one to use is a matter of taste.

The major reason that I use == false is that I have found that !is too easy to overlook, when looking at code.

Having been bitten severely by this, I've made a habit of making it very clear when testing for false.

---

Had the operator been named not as in Pascal, I do not think this would have become an issue.

Answer 5

If condition == false is indeed “much more natural in English” for you then I must assume that you are not a native speaker. Otherwise I cannot explain this, because nobody speaks like that:

If the sun is shining is false I stay at home.

Compare that to

If the sun is not shining I stay at home.

That said, I agree that the single, slender ! character is easily overlooked in code. For that reason, I prefer the keyword not when supported by the language. C++ for instance does allow this although many programmers are not aware of it.

For languages that require !, I put a space between operator and operand. This makes the negation much harder to overlook:

if (! condition) { … }

Notice that every programmer should translate this automatically, without second thought, to “not condition” in their head. Acquiring this kind of fluency in reading code idioms is among the first steps in becoming a good programmer.

Answer 6

because sometimes you might write boolean = false (with the obvious errors) and false == boolean doesn't seem natural (no matter how good of a practice it is)

Answer 7

When I see var == false I always wonder whether var is boolean or of a logic type with more than two values (with, for example, a maybe and an undefined as well as true and false, or something like the nine values of IEEE 1164).

Answer 8

if (!boolean_variable) translates to if the condition is not true.

if (boolean == false) translates to if the condition not false is true. Because that is inversed logic, it is harder to understand.

Answer 9

In (much) older compilers, I believe they would break (boolean == false) into 2 register assignments and a compare code in machine language. The first example would be broken into one assignment and a NOT operator. In terms of performance, the compare operation would take a number of clock cycles, depending on the size of the register being compared, compared to a bitwise invert (1 clock) and would be slower to execute.

That being said, I believe newer compilers do away with this, so it should be OK to go with either.

Answer 10

When you're testing the true condition, it makes sense to do just if (condition), especially when you apply the convention of naming boolean variables beginning with 'is': if (isOpen) is perfectly clear and using != false would be redundant.

For a C/C++/Java/etc. programmer, the meaning of the '!' operator is completely assimilated, to the point that we automatically have 'not' in our minds when we see it. So having if (!isOpen) is as clear as if (_NOT_ isOpen) for me. But you're not familiar enough, in C/C++ you could create a macro with #define _NOT_ !. But trust me, after a few years this is completely unnecessary.

Aside that, it's always preferable to test boolean values without comparing them with literals. For instance, it's dangerous to test if (x == true) because a boolean value is considered true if it's not zero, and the literal true has just one specific value, so x could be 'true' (i.e. nonzero) and still the comparision evaluate to false (because it contains 2 and the literal true is, say, 1.) Of course that doesn't apply to a comparision with false, but if you don't use it when testing for true, why use it when testing for false?

Answer 11

At work I often am dealing with Booleans which can be null so I'll often code this case as

if (value != null && value == true) {
        //do something
}

because I personally feel the symmetry makes it easier to read. Especially if there are other Booleans being tested as well.

I don't really care one way or the other.

Answer 12

Size matters ;)

In mixed expressions it is easier to read:

boolean1 = false
boolean2 = true

p ! boolean1 and ! boolean2
p boolean1 == false and boolean2 == false

And especial for ruby an example where it is a big difference:

boolean = nil
p ! boolean         #-> true
p boolean == false  #-> false

nil is not false, but it is also not true.

Answer 13

Based from my experience and the answers from my quesion that you have linked to.

Some people prefer to use if(condition), for the reason is that one it is shorter to write. and for me it actually makes sense, for example (! isValidated()) I read this as Not Validated. but for me it's all based on personal preferences, and it depends on the logical structure of isValidated() method, if it either returns true or false

Answer 14

If you named your variable properly, then !boolean is more natural. It reads as not boolean to anyone who's enough of a programmer to read code mentally.

Answer 15

For some folks, the sooner a meaning is expressed the better.

For those folks having "if ! ..." compares faster to "if ..." then having to read through the entire condition (which could actually be pretty long, e.g. (thisThing = thatThing or something = the other thing) OR (thinga = thingb and thinga = thingd), etc.) just to find the ==false at the end.

Having the ! (I actually prefer a not when the language allows it) right up front gets the english 'not' for this condition in there sooner.

This issue also leads to consideration for using until in languages that support it, e.g. do 'common stuff' until thing completes. As others say, natural language expression is the goal. I like the "sun is shining" example above.

Answer 16

Another reason is that if you are working in a codebase that uses several languages, if there's one idiomatic way to do something that's safe in all of the languages, it's a pretty good idea to do it that way everywhere so that you form a good habit and are less likely to trip up.

I can't think of anywhere that if (!variable) (or equivalents such as if not variable depending on your language) is not safe, whereas e.g. if self.is_ready() == False: ... is not safe in python if self.is_ready() returns None, now or in the future, which would be a totally reasonable thing for it to do to indicate it wasn't ready since None is just as falsey as False.

Answer 17

In newer languages, like Swift, you have “optional” types, including “optional bool”. An optional bool has values false, true, or nil. In that case, an explicit comparison gives exactly what you ask for. x == false tells you x is false, not true or nil. x != true tells you that x is not true, but either false or nil.

So in Swift, x == false would catch attention, because I will automatically assume that x might be nil.